using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace SilkSharp
{
	/*
	 * Downsample by a factor 2, mediocre quality.
	 * 
	 * @author Jing Dai
	 * @author Dingxin Xu
	 */
	public class Silk_resampler_down2 
	{
		/*
		 * Downsample by a factor 2, mediocre quality.
		 * @param S State vector [ 2 ].
		 * @param S_offset offset of valid data.
		 * @param out Output signal [ len ].
		 * @param out_offset offset of valid data.
		 * @param in Input signal [ floor(len/2) ].
		 * @param in_offset offset of valid data.
		 * @param inLen Number of input samples.
		 */
		public static void SKP_Silk_resampler_down2(
			int[]                           S,         /* I/O: State vector [ 2 ]                  */
			int S_offset,
			short[]                         _out,       /* O:   Output signal [ len ]               */
			int out_offset,
			short[]                         _in,        /* I:   Input signal [ floor(len/2) ]       */
			int in_offset,
			int                             inLen      /* I:   Number of input samples             */
		)
		{
			int k, len2 = inLen>>1;
			int in32, out32, Y, X;

			//assert( Silk_resampler_rom.SKP_Silk_resampler_down2_0 > 0 );
			//assert( Silk_resampler_rom.SKP_Silk_resampler_down2_1 < 0 );

			/* Internal variables and state are in Q10 format */
			for( k = 0; k < len2; k++ ) {
				/* Convert to Q10 */
				in32 = (int)_in[ in_offset + 2 * k ] << 10;

				/* All-pass section for even input sample */
				Y      = in32 - S[ S_offset ];
				X      = Silk_macros.SKP_SMLAWB( Y, Y, Silk_resampler_rom.SKP_Silk_resampler_down2_1 );
				out32  = S[ S_offset ] + X;
				S[ S_offset ] = in32 + X;

				/* Convert to Q10 */
				in32 = (int)_in[ in_offset + 2 * k + 1 ] << 10;

				/* All-pass section for odd input sample, and add to output of previous section */
				Y      = in32 - S[ S_offset+1 ];
				X      = Silk_macros.SKP_SMULWB( Y, Silk_resampler_rom.SKP_Silk_resampler_down2_0 );
				out32  = out32 + S[ S_offset+1 ];
				out32  = out32 + X;
				S[ S_offset+1 ] = in32 + X;

				/* Add, convert back to int16 and store to output */
				_out[ out_offset + k ] = (short)Silk_SigProc_FIX.SKP_SAT16( Silk_SigProc_FIX.SKP_RSHIFT_ROUND( out32, 11 ) );
			}
		}
	}
}
